The vascular endothelium plays an essential role in coordinating diverse circulatory functions such as blood flow, thrombosis, leukocyte trafficking, and even metabolism. Normal endothelial function is characterized by a quiescent cell phenotype that is non-proliferative, non-migratory, and exhibits a cell surface that prevents thrombosis, inflammation, and lipid deposition, thereby resisting atherosclerosis and vascular disease. A key stabilizing stimulus for endothelial quiescence is laminar fluid shear stress (FSS) on the cell surface that is a feature of straight vascular segments. In contrast, curved and branching arteries experience chaotic FSS, called disturbed flow, that dictates a less stable, activated, endothelial phenotype that is more susceptible to atherosclerosis. The mechanisms governing endothelial phenotype in response to fluid shear stress are incompletely understood. In this application, we present data that peroxisome proliferator gamma coactivator-1? (PGC1?), is a fluid shear stress-responsive factor in endothelium that is upregulated with laminar, but not oscillatory FSS. Upregulation of PGC1? is important for the activation of key pathways linked to normal vascular homeostasis such as Klf2, Notch, and eNOS that promote a stable anti-atherosclerotic endothelial phenotype. Exciting pilot data links this effect to upregulation of telomerase reverse transcriptase (TERT) and its extra-nuclear, telomere length-independent, function to stabilize maintain mitochondrial homeostasis in response to laminar FSS. Endothelium lacking TERT activity shows mitochondrial fragmentation and fails to align with flow, a key function needed to resist atherosclerosis. Collectively, these data prompt our central hypothesis that endothelial PGC1?-TERT signaling is required for endothelial and vascular adaptation to shear and normal vascular homeostasis. To investigate this hypothesis, we propose to first determine how PGC1? influences endothelial responses to FSS in vivo using a tamoxifen-inducible Cre/Lox system producing endothelial specific PGC1?-gene excision, in situ confocal microscopy, single-cell RNA-seq, Network Medicine, and the ApoE-/- atherosclerosis model. Similarly, we will use the same strategy with inducible endothelial TERT gene excision. Finally, using cell culture of cells lacking either PGC1? or TERT, we will dissect the mechanisms whereby PGC1?-TERT signaling impacts endothelial FSS responsiveness with a particular focus on FSS-induced PGC1? genome occupancy, mitochondrial and cellular metabolism, endothelial cell flow alignment, and TERT localization to the nucleus vs. mitochondria. Collectively, these studies will provide insight into a new paradigm of endothelial cell responsiveness to FSS and the requirements to maintain a quiescent endothelial monolayer that resists vascular disease. With this information, we should have the requisite insight to design new therapies to alleviate morbidity and mortality from vascular disease.